Axial muscle EMG responses evoked by cutaneous flank nerves in the female rat: effects of spinal transection, steroid hormones, and anesthesia.

In the lateral longissimus muscle (LL) of ovariectomized, female rats anesthetized with low surgical doses of urethane (1.0 g/kg), cutaneous reflexes with similar EMG and response patterns could be elicited from CNS-intact rats and from rats 24 h after complete thoracic spinal cord transection. The probability of eliciting a response to contralateral cutaneous nerve stimulation alone is much lower in rats with complete spinal transections compared to CNS-intact rats. For both CNS-intact and spinal-transected rats, responses to ipsilateral cutaneous nerve stimulation had a shorter latency and required significantly less current on average than responses to contralateral stimulation. The respective currents for eliciting threshold responses to ipsi- and contralateral stimulation are less for CNS-intact than spinal-transected rats. For both CNS-intact and spinal-transected rats, responses to bilateral cutaneous nerve stimulation were inconsistent in the same animal from run to run. With the variability of response at this anesthetic level, no consistent effects of progesterone (acute, i.v.) or estrogen (acute, i.v. and pretreatment, s.c.) were observed in spinal-transected rats. Intravenous progesterone reduced early, unilateral responses in CNS-intact rats anesthetized with 1.0 g of urethane/kg. For both CNS-intact and spinal-transected rats, additional anesthesia during EMG recording produced a gradual decline in response magnitude which could be recovered with a modest increase in stimulus intensity. However, spinal-transected rats appear to require less anesthesia to reduce comparable responses. The results suggest that supraspinal input is especially effective for facilitating contralateral cutaneous reflexes in back muscles, whereas it contributes more equally with afferent input and segmental circuitry to the efficacy of ipsilateral cutaneous reflexes.     

Reflex choroidal blood flow responses of the eyeball following somatic sensory stimulation in rats

The effect of cutaneous mechanical stimulation on choroidal blood flow (ChBF) of the eyeball measured using a laser Doppler flowmeter was examined in anesthetized rats. Noxious pinching stimulation of a forepaw for 20 s produced increases in ChBF and mean arterial blood pressure (MAP), whereas brushing of a forelimb produced no changes in either parameter. After spinal transection at the fourth thoracic (T4) level, forepaw pinching stimulation did not produce any MAP changes in 9 of 11 spinalized rats. In these nine spinalized animals, pinching stimulation of a forepaw produced no significant responses in ChBF. After the cutting of cervical sympathetic trunks in five spinal rats, forepaw pinching showed no effect on MAP, but produced an increase in ChBF, which was abolished by an intravenous (i.v.) injection of 1-(2-trifluoromethylphenyl) imidazole (TRIM), a selective inhibitor of neuronal nitric oxide synthase (nNOS). In another four spinalized rats, whose cervical sympathetic trunks were intact and the superior salivary nucleus (SSN) was destroyed, forepaw pinching showed no effect on MAP, but produced a decrease in ChBF, which was abolished by an i.v. injection of phentolamine, an alpha-adrenoceptor antagonist. The present experiment shows that somatic afferent stimulation can produce reflex responses of the ChBF of the eyeball, either a vasodilative response using parasympathetic efferent fibers or a vasoconstrictive response using sympathetic efferent fibers, independent of systemic blood pressure. It was also shown that the somatically induced vasodilative response was due to a release of nitric oxide (NO) from parasympathetic nerves and the vasoconstrictive response was due to a release of noradrenaline from sympathetic nerves.     

Cutaneous stimulation regulates blood flow in cerebral cortex in anesthetized rats

The effect of noxious or innocuous mechanical stimulation of cutaneous areas (face, forelimb and paw, back, hindlimb and paw) on cerebral blood flow in cortex was examined with laser Doppler flowmetry in anesthetized rats. Pinching of the face, forepaw and hindpaw for 15 s produced significant increases of systemic blood pressure and of cortical blood flow, whereas pinching of the back or brushing of any cutaneous area produced no significant changes in either parameter. Following spinal transection at the first thoracic level, the blood pressure response to forepaw pinching was suppressed, whereas the increase in cortical blood flow still took place. Thus the results suggest that the increase in cortical blood flow following cutaneous noxious stimulation is, in part at least, independent of changes in blood pressure and of any concomitant vasodilatation.     

Electrical stimulation of thalamic Nucleus Submedius inhibits responses of spinal dorsal horn neurons to colorectal distension in the rat

In 78 halothane-anesthetized rats, we characterized the responses of single neurons in the dorsal horn of L(6)-S(1) spinal segments to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of focal electrical stimulation of Nucleus Submedius (Sm) on these responses using standard extracellular microelectrode recording techniques. A total of 102 neurons were isolated on the basis of spontaneous activity. Eighty (78%) responded to CRD, of which 70% had excitatory and 30% had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 100 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 73 of the spinal dorsal horn neurons isolated. Fifty-seven (78%) of these neurons (46 CRD-responsive and 11 CRD-nonresponsive) had cutaneous receptive fields, of which 35 (61%) were small and ipsilateral, 14 (25%) were large and ipsilateral, 7 (12%) were large or small and bilateral, and 1 (2%) was small and contralateral. Sixty-one percent of these neurons responded to both noxious and innocuous cutaneous stimulation, 35% responded only to noxious stimulation, and 4% responded only to innocuous stimulation. Electrical stimulation (50-300 microA) of the contralateral Sm produced intensity-dependent attenuation of the CRD-evoked activities of most neurons (18/28 of CRD-excited and 7/12 of CRD-inhibited) tested. Sm stimulation produced facilitation of CRD responses of only one neuron (CRD-inhibited). Sm stimulation had no effects on spontaneous activity. These data indicate that Sm may be involved in the descending inhibitory modulation of visceral nociception at the spinal level.     

Nonsegmental inhibition of rat dorsal horn neurons by innocuous stimulation

In rats anesthetized with thiamylal sodium, responses of spinal cord dorsal horn neurons to noxious skin heating of the tail were recorded by extracellular microelectrodes. Inhibition of these responses by innocuous mechanical stimulation (light brushing) of the ipsilateral forelimb was assessed. Short-lasting application (3 min, or less) of light brushing did not inhibit neuronal responses to noxious heating. Long-lasting application (5 min, or more) inhibited responses of these neurons to noxious stimulation. The results indicate that, in the anesthetized rat, remotely applied innocuous cutaneous stimuli can inhibit nociceptive responses of dorsal horn neurons, if applied for a sufficiently long time.     

Ovarian blood flow is reflexively regulated by mechanical afferent stimulation of a hindlimb in nonpregnant anesthetized rats

The effects of mechanical afferent stimulation of either a hindpaw or a hind leg on ovarian blood flow and systemic arterial blood pressure were studied in anesthetized nonpregnant adult female rats. Ovarian blood flow at the left side was continuously measured using a laser Doppler flowmeter. A mechanical stimulus (by pinching, brushing or pressing) was delivered for 30 s to either a hindpaw or a hind leg, at the right or left side. Pinching of a hindpaw at the right or left side decreased ovarian blood flow 8+/-3% accompanied with an increase in blood pressure. Brushing or pressing of a hindpaw produced no effects on these two parameters measured. Pressing stimulation of a hind leg at the right or left side produced a decrease in ovarian blood flow 8+/-2% accompanied with a decrease in blood pressure. Pinching or brushing of a hind leg produced no effects on both parameters measured. All responses of ovarian blood flow and blood pressure mentioned above were abolished after severing the femoral and sciatic nerves at the same side in which stimulation was unilaterally delivered. The blood pressure response, either increased or decreased depending on the stimulus modality and location stimulated, was not influenced by severing of ovarian sympathetic nerves. However, the decrease response in ovarian blood flow following pinching of a hindpaw reversed to the increase response, while the decrease response in ovarian blood flow following pressing of a hind leg was slightly augmented. The activity of sympathetic nerves innervating the ovary was increased during pinching of a hindpaw of either side, and decreased during pressing of a hind leg of either side. Electrical stimulation of the distal part of the severed splanchnic nerve at the left side decreased the left ovarian blood flow, and this response was abolished by intravenous injection of phentolamine, the alpha-adrenoceptor antagonist. The present results demonstrate that ovarian blood flow increase or decrease passively to responses of an increase or a decrease in systemic arterial blood pressure following mechanical afferent stimulation of a hindlimb, and these passive responses of ovarian blood flow are modulated by reflexive activity of ovarian sympathetic vasoconstrictor nerves to attenuate blood pressure-dependent passive changes in ovarian blood flow.     

Hepatic blood flow responses to mechanical stimulation of the skin in anaesthetised rats

The aim of the present study was to investigate how hepatic blood flow (HBF) changes in response to mechanical stimulation of different areas of the skin in anaesthetised rats, by focusing on involvement of the hepatic sympathetic nerves in and contribution of systemic circulatory changes to the HBF responses. HBF was measured at the surface of the left lateral lobe using the laser Doppler flowmetry. Both innocuous and noxious mechanical stimuli were applied to skin areas of the abdomen and hindlimb. Innocuous mechanical stimulation (brushing) of the abdomen and hindlimb did not significantly change HBF, while noxious mechanical stimulation (pinching) of the abdomen and hindlimb did. The responses to pinching were dependent on the sites stimulated. Pinching of the abdomen decreased, while pinching of the hindlimb increased the HBF. The decrease of HBF in response to abdominal pinching remained after the spinal cord was transected at T1-2 level, but the response was diminished after hepatic sympathetic nerves were severed. On the other hand, the increase of HBF in response to hindlimb pinching was dependent on the increase in blood pressure, and was not influenced by the severance of hepatic sympathetic nerves, and the responses to hindlimb pinching were almost absent after the spinal cord was transected. Based on these results, we suggest that noxious mechanical stimulation of the skin produces changes of HBF, either as a reflex response via activation of the hepatic sympathetic nerves or as a passive response to systemic circulatory changes, depending on the sites stimulated.     

Influence of innocuous cervical vertebral movement on the efferent innervation of the adrenal gland in the rat

In general, in central nervous system intact anesthetized animals, adrenal sympathetic efferent nerve activity and catecholamine secretion increase in response to noxious somatic stimulation, and decrease in response to innocuous somatic stimulation. In anesthetized rats, noxious chemical stimulation of the thoracic and lumbar interspinous tissues is associated with large increases in adrenal sympathetic efferent nerve activity and catecholamine secretion, with a clear segmental organization to the reflex apparent in spinalized animals. However, the adrenal sympathetic nerve responses to mechanical stimulation in the form of pressure applied laterally to the lower thoracic and lower lumbar vertebrae do not display segmental organization, and the depressor response is more characteristic of responses to innocuous somatic stimulation despite the use of large forces (up to 3.0 kg). Therefore, we sought to determine whether innocuous movements of the mechanoreceptor-rich deep tissues of the neck modulate the sympathetic outflow to the adrenal gland. We performed experiments in 14 anaesthetised (Urethane 1 g/kg and Chloralose 0.1 g/kg) adult rats. Rats were intubated and breathed spontaneously. A computer driven small animal manipulator was used to impose ramp and hold rotational displacements (12 degrees /s, hold duration 2 s) of the 2nd cervical vertebra (range 2-30 degrees ) while recording multi-unit activity from sympathetic nerves innervating the adrenal gland. While noxious forepaw pinch elicited an increase in sympathetic nerve activity to the adrenal gland, there was no significant change in sympathetic nerve activity with small (2 degrees or 6 degrees ) rotations. Significant changes (P<0.05) in sympathetic activity were observed in only 7% (n=21) of all trials at larger displacements (12 degrees , 20 degrees , 25 degrees , 30 degrees n=287 trials). Our data suggest that although noxious stimuli may modulate sympathetic outflow, it is rare for afferents signalling innocuous cervical vertebral movements to modulate sympathetic nerves innervating the adrenal gland.     

Somatostatin: evidence for a role in thermal nociception

In barbiturate-anaesthetized spinalized cats, antibody microprobes were used to investigate the release of immunoreactive somatostatin (irSS) in the lumbar dorsal horn in response to cutaneous stimuli. In the absence of applied stimulation, a significant basal release of irSS was present in the region of the substantia gelatinosa. Such release was not increased by innocuous or noxious cutaneous mechanical stimuli nor by innocuous thermal stimuli, but was increased by noxious thermal stimulation. The magnitude of this noxious heat-evoked release was estimated by comparing in vivo microprobes with those used to detect known concentrations of somatostatin in vitro. Pairs of microprobes were used to detect simultaneous release of both irSS and immunoreactive substance P in the substantia gelatinosa. The results support the putative role of somatostatin in the spinal transmission of thermal nociceptive information.     

Altered properties and laminar distribution of neuronal responses to peripheral stimulation in the Sml cortex of the arthritic rat

The properties of the neuronal responses to different types of mechanical peripheral stimulation were studied during electrode penetrations in the first somatosensory cortex of anaesthetized rats with polyarthritis. Very few neurons were driven by light cutaneous stimulation (such as brushing) or by intense mechanical stimulation. Most of the neurons were driven by joint movement and/or moderate pressure on the skin. These neurons could be found in all cortical layers, the majority being located in layer V. These results contrast sharply with the properties and laminar distribution of the different functional categories of cortical neurons, as observed in normal animals.     

Electrophysiological properties of ventromedial medulla neurons in response to noxious and non-noxious stimuli in the awake, freely moving rat: a single-unit study

The spontaneous and evoked activities of ventromedial medulla (VMM) neurons have been recorded in the chronic, awake, freely moving rat. The vast majority of neurons located at the level of the nucleus raphé magnus exhibited an irregular and variable (2-16 Hz) spontaneous activity and were activated by either cutaneous or auditory stimuli. Within this convergent neuronal class the neurons were activated by either cutaneous noxious and non-noxious inputs. The threshold for cutaneous activation was likely very low since a majority of units responded to air puffs, but the application of controlled brushing and pin-prick revealed that the VMM convergent neurons responded more for the noxious mechanical stimulation. Similar findings were found with pinch application. For both innocuous and noxious stimuli, the cutaneous receptive field was extremely extensive (almost all of the body); however, the application of the controlled brushing showed that for this innocuous stimulation, the most sensitive regions were the tail, back, snout and vibrissae and, to a lesser extent, the flank and paws. Preliminary experiments indicated that both the spontaneous and evoked activities of VMM convergent neurons were inhibited during stressful manipulations such as scruff lifting or defense reactions. These data contrast with other studies on VMM single unit recordings in anesthetized rats since the majority of these studies did not emphasize the VMM convergent group; in addition, with one exception, we did not find neurons exclusively driven by noxious inputs. Without excluding a role of the VMM convergent group in pain descending control systems, we proposed that this neuronal class is perhaps also involved in pain transmission or in general processess such as alertness and stress. Experiments are proposed in order to precisely determine the involvement of the VMM convergent neurons in alertness versus sensory discriminative aspects of nociception in the awake, freely moving rat.     

Effect of capsaicin on micturition and associated reflexes in chronic spinal rats

The role of capsaicin-sensitive bladder afferents in micturition was studied in unanesthetized chronic spinal rats. Reflex voiding in response to tactile stimulation of the perigenital region appeared 5–9 days after spinal cord injury (SCI) whereas voiding induced by bladder distension occurred 2–3 weeks after SCI. The frequency and amplitude of reflex bladder contractions recorded under isovolumetric conditions were similar in chronic spinal and urethane-anesthetized CNS-intact rats. However, cystometrograms (CMGs) performed 6–8 weeks after SCI revealed that the chronic spinal rats had larger bladder capacities (1.86 ml) than CNS-intact rats (0.48 ml) and also exhibited multiple, small-amplitude, nonvoiding bladder contractions that were not detected in CNS-intact rats. Administration of capsaicin (50 mg/kg s.c.) acutely (onset 14–40 min) suppressed reflex bladder activity induced by bladder distension or by perigenital stimulation in chronic spinal animals. However, pretreatment of chronic spinal rats with capsaicin (125 mg/kg s.c.) 4 days before the experiment did not depress voiding reflexes or change bladder capacity but did eliminate the nonvoiding contractions. Inhibition of reflex bladder contractions by mechanical stimulation of rectoanal canal or the uterine cervix-vagina was not altered by pretreatment with capsaicin. These data indicate that capsaicin-sensitive bladder afferents are not essential for the initiation of reflex micturition in chronic spinal rats. However, these afferents do contribute to hyperactivity of the bladder during the filling phase of the CMG. Thus, capsaicin-sensitive bladder afferents should be evaluated as possible targets for the pharmacological treatment of bladder hyperreflexia in patients with SCI.     

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.     

Dorsolateral funiculus-lesions unmask inhibitory or disfacilitatory mechanisms which modulate the effects of innocuous mechanical stimulation on spinal Fos expression after inflammation

To examine the contribution of low threshold mechanoreceptive afferent input to the development of allodynia and the involvement of descending pathways, we investigated the effects of repeated innocuous brush on inflammation-induced spinal Fos protein expression in dorsolateral funiculus-lesioned (DLFX) rats following hindpaw inflammation. In DLF sham-operated animals, brush stimuli induced a significant increase in the number of Fos-labeled neurons in ipsilateral laminae I–IV, and a slight suppression of Fos expression in ipsilateral laminae V–VI when compared to sham-lesioned rats without brushing. In rats receiving DLFX, the brush-induced increase in Fos expression in laminae I–IV was significantly reduced. The DLFX also unmasked a brush-induced suppression of laminae VII–VIII neurons. These results suggest that innocuous mechanical stimulation of an inflamed hindpaw gives rise to further facilitation of neuronal activity in laminae I–IV and inhibition of neuronal activity in laminae V–VIII. We propose that there is an unmasking of inhibitory mechanisms or a reduction in descending facilitatory effects after DLFX that alter Fos protein expression produced by innocuous mechanical stimulation.     

Responses of neurons in the nucleus basalis of Meynert to various afferent stimuli in rats

The effects of innocuous and noxious mechanical stimulation of skin, and of baroreceptor and chemoreceptor stimulation, on the activity of single neurons in the nucleus basalis of Meynert (NBM), whose axons project to the cortex, were examined in urethane-anesthetized adult rats. Most of the neurons were not significantly influenced by innocuous mechanical cutaneous stimulation or baroreceptor stimulation, while they were excited by noxious mechanical cutaneous stimulation and chemoreceptor stimulation. The NBM neurons were excited more intensely and frequently by nociceptive mechanical stimulation to a fore- or hindpaw than by that to the back or face. The function of these NBM neurons is discussed.     

The effect of morphine on responses of nucleus ventroposterolateralis neurons to colorectal distension in the rat

In 71 halothane-anesthetized rats, we characterized the responses of single neurons in the nucleus ventroposterolateralis (VPL) of the thalamus to a noxious visceral stimulus (colorectal balloon distension; CRD) and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred nine neurons were isolated on the basis of spontaneous activity. Sixty-four (59%) responded to CRD, of which 52 (81 %) had excitatory and 12 (19%) had inhibitory responses. Neurons showed graded responses to graded CRD pressures (20-100 mmHg), with maximum excitation or inhibition occurring at 80 mmHg. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 95 of the VPL neurons isolated. Eighty-three of these neurons (48 CRD responsive and 35 CRD nonresponsive) (87%) had cutaneous receptive fields, of which 96% were small and contralateral and 4% were large and contralateral or bilateral. Ninety-four percent of these neurons responded to both noxious and innocuous cutaneous stimulation, and 6% responded to only noxious stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.125, 0.25, 0.5, 1, and 2 mg/kg, i.v) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/ kg, i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VPL neurons in visceral nociception and are consistent with a role of VPL in sensory-discriminative aspects of nociception.     

Oxytocin actions on afferent evoked spinal cord neuronal activities in neuropathic but not in normal rats

A hypothalamic oxytocinergic-descending pathway that reaches the dorsal horn of the spinal cord has been well documented and recently related to states of pain and analgesia. In order to study the action of the neuropeptide oxytocin (OT) on pain-related responses, we compared dorsal horn neuronal responses to electrical and mechanical stimulation of receptive fields in normal and neuropathic rats. Spinal nerve (L5 and L6) ligation (Chung rats) was used to produce experimental neuropathy. Single unit activity was recorded at the L4-L5 level from neurons identified as wide dynamic range presenting latency responses corresponding to A-beta, A-delta, C fibers and also exhibiting post-discharge, and wind-up. We tested intrathecally applied doses of 0.05, 0.1, 1, 2, 5, 10 I.U. of OT. Minor effects on responses to electrical stimulation were present in normal rats. Mechanical responses evoked by von Frey filaments were slightly reduced in normal animals. In neuropathic rats a dose of 1 I.U. produced a significant reduction in C-fibers and post-discharge activities, and doses of 2 I.U. caused a further, pronounced reduction in post-discharge, wind-up, and input values. However, the most marked change was the post-discharge reduction at 10 and 20 min after OT administration. Mechanical responses were significantly reduced in terms of their discharge rate response in neuropathic rats. The contrasting results obtained in normal and neuropathic rats revealed an important distinction between these animals and indicate that plastic changes occur as a consequence of nerve damage. In neuropathic rats, mechanisms involving ascending noxious information to the paraventricular nuclei and descending OT activities could be altered so sensitizing the OT receptors of the spinal dorsal horn cells and could explain our observations. Our results point out an anti-algesic OT effect in neuropathic rats.     

The effect of morphine on responses of ventrolateral orbital cortex (VLO) neurons to colorectal distension in the rat

In 49 halothane-anesthetized rats, we characterized the responses of single neurons in the ventrolateral orbital cortex (VLO) to a noxious visceral stimulus (colorectal balloon distension, CRD), and studied the effects of intravenous morphine on these responses using standard extracellular microelectrode recording techniques. One hundred and four neurons were isolated on the basis of spontaneous activity. Fifty-seven (55%) responded to CRD, of which 32% had excitatory and 68% had inhibitory responses. Neurons showed tendencies toward graded responses to graded CRD pressures (20–100 mmHg), with maximum excitation or inhibition occurring at 80 or 100 mmHg, respectively. Responses to noxious (pinch, heat) and innocuous (brush, tap) cutaneous stimuli were studied in 80 of the VLO neurons isolated. Thirty-three (41%) of these neurons (21 CRD-responsive and 12 CRD-nonresponsive) had cutaneous receptive fields, of which 79% were large and bilateral, 18% were small and bilateral, 3% were small and ipsilateral. Ninety-four percent of these neurons responded only to noxious cutaneous stimulation, 6% responded to both noxious and innocuous stimulation. No neurons responded solely to innocuous stimulation. Cumulative doses of morphine (0.0625, 0.125 and 0.25 mg/kg i.v.) produced statistically significant dose-dependent attenuation of neuronal responses to CRD. Naloxone (0.4 mg/kg i.v.) reversed the effects of morphine. Morphine and naloxone had no significant effects on spontaneous activity. These data support the involvement of VLO neurons in visceral nociception.     

Glutamate uptake is attenuated in spinal deep dorsal and ventral horn in the rat spinal nerve ligation model

Alteration of glutamatergic (GLU) neurotransmission within the spinal cord contributes to hyperalgesic and allodynic responses following nerve injury. In particular, changes in expression and efficacy of glutamate transporters have been reported. Excitatory, pain transmitting primary afferent neurons utilizing glutamate as an excitatory neurotransmitter project to both superficial (I-II) and deep (III-V) laminae of the dorsal horn. These experiments were designed to examine changes in glutamate uptake occurring concomitantly within the spinal deep dorsal and ventral horn in situ after experimentally induced neuropathic pain. In vivo voltammetry, using microelectrode arrays configured for enzyme-based detection of GLU were employed. Sprague-Dawley rats had either sham surgery or tight ligation of L5 and L6 spinal nerves (SNL). Four to six weeks later, the L4-L6 spinal cord of chloral hydrate-anesthetized animals was exposed, and ceramic-based glutamate microelectrodes equipped with glass micropipettes 50 microm from the recording surfaces were placed stereotaxically at sites within the spinal cord. Pressure ejection of GLU into the ipsilateral L5-L6 spinal cord resulted in a 72% reduction of GLU uptake in SNL rats compared to sham controls in the ipsilateral L5-L6 deep dorsal horn and a 96% reduction in the ventral horn. In contrast, in the same animals, the contralateral L5-L6 or the ipsilateral L4 spinal cord showed no change in glutamate uptake. The data suggest that spinal nerve ligation produced attenuated glutamate uptake activity extending into the deep dorsal and ventral horn. The study suggests that plasticity related to spinal nerve injury produces widespread alteration in glutamate transporter function that may contribute to the pathophysiology of neuropathic pain.     

Ventral root potentials of the cat's sacrococcygeal segments evoked by stimulation of intact and transected dorsal roots

The latency and amplitude of reflex-evoked potentials in the sacrococcygeal ventral roots of acute spinalized cats were investigated. The characteristics of the potentials were examined in response to electrical stimulation of intact and acutely transected dorsal roots. We found that: the last sacral and caudal (coccygeal) segments of the cat's spinal cord are endowed with electrophysiologic characteristics that distinguish them from other spinal segments (e.g., L7-S1); afferent stimulation of the corresponding intact dorsal roots evokes in the ventral root of segment S2 a small monosynaptic response, whereas no monosynaptic response is seen in segment Ca6; acute transection of the dorsal roots provokes an increment of the monosynaptic response in all segments studied except for Ca6; rhizotomy provokes in Ca5 the appearance of polysynaptic responses to electrical stimulation of the corresponding dorsal root; and transection of the cutaneous afferent fibers of the coccygeal motoneurons resulted in an increment of monosynaptic and polysynaptic responses, indicating the removal of inhibitory effects.